Regarding technologies for expanding an audio signal having a narrow frequency band into a broadband, various propositions have been made to date.
For example, Japanese Unexamined Patent Application Publication No. 7-36490 (hereinafter referred to as Document 1) discloses a method in which, based on an input audio signal spectrum, lost harmonic components are generated by an arithmetic operation and are added.
Also, in a paper from Saitama University to the Acoustical Society of Japan, a method has been announced in which a phonic sound source waveform is generated by an autocorrelation arithmetic operation on an input signal and is used as high frequency components.
In addition, Japanese Unexamined Patent Application Publication No. 11-126097 and Japanese Unexamined Patent Application Publication No. 11-68515 (hereinafter referred to as Documents 3) discloses a sample-value interpolating method in the case of performing D/A conversion at a higher sampling frequency compared with an input signal.
Moreover, Japanese Unexamined Patent Application Publication No. 6-118995 (hereinafter referred to as Document 4) discloses a method that restores a broad-range audio signal by establishing correlation between a broad-range-audio-signal vector-quantization codebook and a narrow-range-audio-signal vector-quantization codebook.
The technology in Document 1 is such that, in a relatively-broad-range audio signal as in MP3 (Moving Picture Experts Group-1, Audio Layer-3) or the like, a high frequency signal which is lost in the process of compressing the audio signal is predicted and added, and its application is difficult, for example, when the bandwidth of an extremely-narrowed-range signal, such as a telephone audio range, is doubled.
Also, the technology in Document 2 is a process only on voiced sound and has a problem in that unvoiced sound (consonant portion) cannot be restored.
The technology in Documents 3 is such an extension of an interpolating filter that, in the case of sample value interpolating processing for increasing a sampling frequency, the level of the interpolating sample value is set to a value which is considered as appropriate after observing circumstances in which several adjacent samples change, and thus does not restore lost frequency components.
The technology in Document 4 is considered as assuming a vocal tract model as an input signal since an LPC (Liner Prediction Coefficient) analysis and synthesizing processing are used as a processing core. Accordingly, its application to musical sound is difficult.